Vehicle transmission systems

ABSTRACT

A transmission system for a four wheel drive vehicle includes an input shaft ( 2 ) connected to a differential mechanism ( 4, 6, 10 ), which has two output shafts ( 12, 20 ), which are connected, in use, to respective pairs of driven wheels of the vehicle. The two output shafts carry first and second coaxially mounted sun wheels (S 1 , S 2 , respectively), of an epicyclic gear system which mesh with first and second sets of planet wheels (P 1 , P 2 , respectively), the epicyclic gear system including a third sun wheel (S 3 ), which is mounted coaxially with the first and second sun wheels and is in mesh with a third set of planet wheels (P 3 ). The gear ratios of the three meshing sun wheels and sets of planet wheels are different. Each first planet wheel (P 1 ) is connected to respective second and third planet wheels (P 2 , P 3 ) to rotate therewith about a respective common planet shaft, the planet shafts being connected to a common carrier ( 24 ) which is rotatably mounted coaxially with the three sun wheels. The carrier is connected to one side of a first clutch ( 28 ), the other side of which is connected to a fixed structure. The third sun wheel is connected to one side of a second clutch ( 34 ), the other side of which is connected to a fixed structure. The transmission system also includes at least one sensor ( 38 ) arranged to produce a signal indicative of an operating parameter of the vehicle or its engine and a controller ( 36 ) connected to the sensor and to the two clutches and arranged to operate the clutches in response to the said signal.

[0001] The present invention relates to vehicle transmission systems andis concerned with transmission systems for four wheel drive vehicles.

[0002] Automotive differentials are of course well known and include asingle input shaft, normally connected to the output shaft of thevehicle gearbox, and two output shafts which are permitted to rotate atdifferent speeds. The mean of these two speeds is directly proportionalto the input speed. When a vehicle travels on a radius, that is to saygoes round a corner, the outer wheels travel further than the insidewheels in the same time and must therefore rotate faster. The basicdifferential caters for this speed difference but the torque supplied toboth wheels remains the same. However, there are many circumstances inwhich it is desirable for the torque supplied to the two wheels todiffer. Thus if one wheel is on a low friction surface, e.g. ice, andthe other on a high friction surface, e.g. a dry patch of roadway, themaximum torque that can be transmitted to the two wheels by aconventional differential is limited to the very low torque value whichmay be transmitted to that wheel which is in contact with the ice. Thepotential propulsive force available through the wheel in engagementwith the high friction surface cannot be used and the propulsive forceexerted may therefore be insufficient to move the vehicle. A vehiclecornering with a high lateral acceleration has a significant weighttransfer from the inside wheels onto the outer wheels. As a result ofthe reduced weight on the inside wheels, only a low torque can betransmitted through the inside driven wheel before wheel slip occurs andthis means that, with a conventional differential. only the same lowtorque may be transmitted to the outer driven wheel. This severelylimits the potential acceleration of the vehicle, when cornering. Inorder to overcome these disadvantages, numerous devices are known tolock or limit slip in the differential, whereby different levels oftorque may be transmitted through the two output shafts.

[0003] In the case of a four wheel drive vehicle, there isconventionally a differential between the front and rear axles toaccommodate the speed differences between them whilst transmitting powerto both axles. In this case also there are many circumstances underwhich a simple differential is inadequate. A four wheel drive facilityis generally provided on vehicles with an excess of power and superiorperformance could therefore be achieved by transmitting the maximumamount of power through all the wheels. The maximum rate of accelerationcan only be achieved if the torque is proportioned between the wheels tomatch the weight distribution of the vehicle. As a result of dynamicweight transfer, e.g. under heavy acceleration, as much as 80% of theweight may be carried by the rear wheels An unequal torque distributioncould be achieved by providing a differential with an invariable torquesplit to match this proportion but when cornering or driving on an icyroad this torque split ratio could result in vehicle instability andthus loss of control. Under these conditions it would be advantageousfor a higher proportion of the torque to be provided to the front axle.

[0004] In the case of “off road” vehicles, which frequently have a shortwheel base and a high centre of gravity, the weight transfer whenclimbing a steep gradient could be as much as 90% onto the rear axle. Alimited slip type of differential could deliver sufficient traction butcould also cause slippage of the front wheels, thereby leading to lossof stability. Locking the centre differential, that is to say thedifferential between the front and rear axles, would prevent front wheelslip but reduces manoeuvrability which is a significant problem on thepoor surfaces which are commonly encountered when driving off road.

[0005] When descending a steep gradient, the safest technique is to usea low gear ratio and rely solely or principally on engine braking tomaintain control. Under these circumstances, the weight transfer thatoccurs may place as much as 90% of the vehicle weight onto the frontaxle.

[0006] It is therefore the object of the invention to provide atransmission system for four wheel drive vehicles which will split theengine torque between the front and rear axles in a proportion which isvariable in dependence on one or more operating. parameters of thevehicle or its engine rather than merely limiting the slip of or lockingthe differential.

[0007] According to the present invention, there is provided atransmission system for a four wheel drive vehicle including an inputshaft connected to a differential mechanism, which has two outputshafts, which are connected, in use, to respective pairs of drivenwheels of the vehicle, the two output shafts carrying first and secondcoaxially mounted sun wheels, respectively, of an epicyclic gear systemwhich mesh with first and second sets of planet wheels, respectively,the epicyclic gear system including a third sun wheel, which is mountedcoaxially with the first and second sun wheels and is in mesh with athird set of planet wheels, the gear ratios of the first sun wheel withthe first set of planet wheels, the second sun wheel with the second setof planet wheels and the third sun wheel with the third set of planetwheels being different, each first planet wheel being connected torespective second and .third planet wheels to rotate therewith about arespective common planet shaft, the planet shafts being connected to acommon carrier which is rotatably mounted coaxially with the first,second and third sun wheels, the carrier being connected to a firstselectively operable speed changing device, the third sun wheel beingconnected to a second selectively speed changing device, thetransmission system further including at least one sensor arranged toproduce a signal indicative of an operating parameter of the vehicle orits engine and a controller connected to the sensor and to the two speedchanging device and arranged to operate the speed changing devices inresponse to the said signal. The present invention also embraces a fourwheel drive vehicle incorporating such a transmission system.

[0008] Thus a four wheel drive vehicle in accordance with the presentinvention will typically include not only a front differential splittingspeed and/or torque between the two front driven wheels, a reardifferential splitting speed and/or torque between the two rear drivenwheels and a centre differential splitting speed between the front andrear axles but also an epicyclic gear system, which is connected to thetwo outputs of the centre differential and includes two speed changingdevices which are actuated by a controller in response to a signalreceived from one or more sensors which detect respective operatingparameters of the vehicle or its engine to split the engine torquebetween the front and rear axles in a proportion which is the optimumfor the prevailing operating conditions.

[0009] Thus in the transmission system in accordance with the inventionthere are two speed chancing devices which are arranged to change thespeed of the carrier and the third sun wheel, respectively, therebyvarying the proportion of the engine torque supplied to the front andrear axles of the vehicle. The speed changing means may be arranged toincrease or decrease the speed of the carrier and the third sun wheel.They may therefore constitute e.g. electric motors. It is, however,preferred that the speed changing devices are arranged to reduce thespeed of rotation of the carrier and the third sun wheel and may thusconstitute simple braking mechanisms. It is, however, preferred thatthey constitute clutches.

[0010] Further features and details of the invention will be apparentfrom the following description of two specific embodiments oftransmission systems in accordance with the invention which is given byway of example only with reference to the two accompanying highlydiagrammatic drawings.

[0011]FIG. 1 shows a transmission system with an input shaft 2 which, inuse, will be connected to or constitute part of the output from thegearbox of the vehicle engine. The shaft 2 is connected to the annulus 4of an epicyclic differential. Formed on the interior of the annulus 4 isan annular array of gear teeth with which the teeth on a number ofplanet gears 6 mesh. The planet gears 6 are carried by a common carrier8 and mesh with a sun gear 10. The carrier 8 is connected to a firsthollow output shaft 12 which is connected to the front axle of thevehicle, not shown, by way of a gear train 14, 16, 18. The sun gear 10is carried by a shaft 20 which is connected to the rear axle, typicallythrough the rear differential.

[0012] The output shafts 12 and 20 are connected to a further epicyclicdifferential gear system which splits the torque transmitted through thetwo output shafts in a manner appropriate to the operating conditions ofthe vehicle. The front wheel output shaft 12 carries a sun gear SI andthe rear wheel output shaft 20 carries a coaxial sun gear S2. Mountedconcentrically with these two sun gears is a further sun gear S3. Thethree sun wheels S1, S2 and S3 are in mesh with respective sets of threeplanet wheels P1, P2 and P3. The various planet wheels are associatedwith one another in three groups, each group including a first planetwheel P1, a second planet wheel P2 and a third planet wheel P3,connected together to form a composite unit. Each composite planet unitP1, P2, P3 is mounted to rotate about a respective planet shaft 22. Thethree planet shafts 22, which extend parallel to the output shafts 12and 20, are connected to a common carrier 24. Projecting radially fromthe carrier 24 are one or more radial clutches 26 which constitute onehalf of a first selectively actuable clutch 28, referred to as the rearbias clutch, the other half of which constitutes or is connected to afixed structure 30, e.g. the outer casing of the epicyclic torquesplitting differential system.

[0013] The sun gear S3 is also connected to one or more radial clutchdiscs 32 which constitute one half of a second selectively operableclutch 34, referred to as the front bias clutch, the other half of whichis also connected to or constituted by the fixed structure 30.

[0014] The two clutches 28, 34 are connected to a power source (notshown) and may be operated independently under the control of anelectronic controller 36. Connected to the controller is a plurality ofsensors, of which only one, 38, is shown schematically arranged tosense, and produce a signal representative of, a number of operatingparameters of the vehicle and/or its engine, such as the vehicle speed,the steering wheel angle, the acceleration of the vehicle in each ofthree orthogonal directions, engine speed, engine torque, yaw rate,which engine gear is engaged, the differential output speed and torquesupplied to the front axle and the differential output speed and torquesupplied to the rear axle.

[0015] In a specific embodiment, the sun wheels S1, S2 and S3 had 23, 21and 21 teeth, respectively, whilst each planet wheel P1, P2 and P3 had12, 12 and 24 teeth, respectively. The gear ratios of each meshing sunwheel and set of planet wheels are therefore all different.

[0016] It will be appreciated that, in use, application of the frontbias clutch 34 will slow the sun gear S3 which induces a ratio changeacross the sun gears S1 and S2, the ratio being (P2/S2)/(P1/S1). This inturn produces a torque transfer from the rear output shaft 20 to thefront output shaft 12. When the rear bias clutch is applied the reverseoccurs and the carrier 24 is slowed which induces a ratio change acrossthe sun gears S1 and S2, the ratio being (P3/S3-P1/S1)/ (P3/S3-P2/S2).There is thus a torque transfer from the front output shaft 12 to therear output shaft 20. This small ratio change produces a small tyre slipwhich is responsible for the torque difference between the axles. Forexample, a tyre slip difference of less than 0.5% produces a torque biasratio of 11.5, i.e. 92% to one axle and 8% to the other (i.e. a biasratio of 92/8). By using pre-programmed algorithms, the controller canproportion the torque between the front and rear axles to optimise thevehicle performance.

[0017] The transmission system described above has essentially a“north-south” configuration, that is to say the transmission inputextends in the fore- and -aft direction of the vehicle parallel to thedrive shaft to the front and rear axles. However, the present inventionis also applicable to a transmission system of “east-west” configurationin which the transmission inlet shaft extends in the transversedirection of the vehicle perpendicular to the drive shaft to the rearaxle. Such a transmission system is illustrated in FIG. 2, in which thesame reference numerals as in FIG. 1 are used to designate similarcomponents.

[0018] The transmission input shaft 2 transmits torque via gear wheel 40to the annulus 4 of the centre epicyclic differential with which theteeth of a number of planet wheels 6 mesh. The output shaft 12 for thefront wheels is connected to rotate with the sun wheel 10 and isconnected to a standard differential 44 which splits the speed and/ortorque between the output shafts 46 and 48 of the two driven frontwheels. The output shaft 20 for the rear wheels is connected to thecarrier of the planet wheels 6 and to the drive shaft 49 for the rearwheels via gear wheels 50, 52, 54 and 56. In other respects, theconstruction of this embodiment is the same as that of the firstembodiment described above in connection with FIG. 1 and will thereforenot be described again.

[0019] It will be appreciated that the ability to vary the split oftorque between the front and rear axles of a four wheel drive vehicle inaccordance with operating conditions, particularly in accordance withthe division of the vehicle weight between the front and rear axles,will produce numerous advantages. These include maximising the potentialacceleration of the vehicle by continuously varying the distribution ofthe torque between the front and rear axles to match the distribution ofthe static and dynamic weight of the vehicle between those axles. Theability of the vehicle to ascend or descend a gradient, either forwardsor backwards, may be optimised by varying the distribution of torquebetween the front and rear axles to match the weight distribution. Thecornering performance of the vehicle may also be optimised by varyingthe torque distribution between the front and rear axles to optimise thetyre slip angle, the side forces and the traction characteristics whilstentering, passing through and leaving a corner. The vehicle stabilitymay also be maximised, especially under high speed and adverse weatherconditions.

[0020] Although only the front or rear bias clutch would be engaged atany one time, it is also possible to engage both clutches simultaneouslyand this will effectively lock the centre differential and constitute ananti-theft device for the vehicle or provide a “hill hold” facility. Thecontroller may be arranged to release both the clutches once the inputtorque exceeds the sum of the wind-up torque holding the front and rearaxles. Immediately after release of the clutches, the controller may bearranged to apply the rear bias clutch to direct torque preferentiallyto the rear axle, if appropriate. This facility is, however, notrequired when the vehicle is facing downhill, unless reverse gear isselected.

[0021] Due to the fact that a significant proportion of the weight of avehicle is shifted forwardly onto the front axle during braking, thefront wheel brakes of a vehicle need to have a performance capability ofe.g. two to three times that of the rear wheel brakes. However, it willbe appreciated that the ability of the transmission system in accordancewith the present invention to transfer torque between the front and rearaxles under drive conditions can be used to transfer torque between thetwo axles under braking conditions also. This facility will enable therear brakes to be sized to match the front brakes, which is currentlynot necessary, and thus enable the vehicle to achieve a substantiallyenhanced braking capability by comparison with known vehicles.

1. A transmission system for a four wheel drive vehicle including aninput shaft connected to a differential mechanism, which has two outputshafts, which are connected, in use, to respective pairs of drivenwheels of the vehicle, the two output shafts carrying first and secondcoaxially mounted sun. wheels, respectively, of an epicyclic gear systemwhich mesh with first and second sets of planet wheels, respectively,the epicyclic gear system including a third sun wheel, which is mountedcoaxially with the first and second sun wheels and is mesh with a thirdset of planet wheels, the gear ratios of the first sun wheel with thefirst set of planet wheels, the second sun wheel with the second set ofplanet wheels and the third sun wheel with the third set of planetwheels being different, each first planet wheel being connected torespective second and third planet wheels to rotate therewith about arespective common planet shaft, the planet shafts being connected to acommon carrier which is rotatably mounted coaxially with the first,second and third sun wheels, the carrier being connected to a firstselectively operable speed changing device, the third sun wheel beingconnected to a second selectively speed changing device, thetransmission system further including at least one sensor arranged toproduce a signal indicative of an operating parameter of the vehicle orits engine and a controller connected to the sensor and to the two speedchanging devices and arranged to operate the speed changing devices inresponse to the said signal.
 2. A transmission system as claimed inclaim 1 in which the first and second speed changing devices arearranged to reduce the speed of rotation of the carrier and the thirdsun wheel, respectively.
 3. A transmission system as claimed in claim 2in which the first speed changing device comprises a clutch, one side ofwhich is connected to the carrier and the other side of which isconnected to a fixed structure, and the second speed changing devicecomprises a clutch, one side of which is connected to the third sunwheel and the other side of which is connected to a fixed structure. 4.A transmission system as claimed in claim 1 in which the differentialmechanism comprises an epicyclic gearset including an annulus in meshwith a set of planet wheels, which are carried by a carrier and are alsoin mesh with a sun wheel, the carrier and the sun wheel being connectedto the two output shafts.